This set of Wireless & Mobile Communications Quiz focuses on “Combined Linear and Constant Envelope Modulation Techniques”.
1. Which of the following is a combined linear and constant envelope technique?
Explanation: M-ary phase shift keying (MPSK) is a combined linear and constant envelope technique. It is a part of M-ary modulation techniques. These modern modulation techniques exploit the fact that digital baseband data may be sent by varying both the envelope and phase of a RF carrier.
2. In an M-ary signalling scheme two or more bits are grouped together to form a _______
Explanation: In an M-ary signalling scheme two or more bits are grouped together to form symbols. And one of the M possible signals is transmitted during each symbol period of duration Ts.
3. The number of possible signal in M-ary signalling is given by M and M=_____, where n is an integer.
Explanation: Two or more bits are grouped to form a symbol in M-ary modulation. And the number of possible symbols should be equal to 2n, where n is an integer.
4. M-ary signalling techniques are not sensitive to timing jitters. State whether True or False.
Explanation: Timing errors increase when smaller distances between signals in the constellation diagram are used. M-ary signalling techniques are attractive for use in bandlimited channel, but are limited in their applications due to sensitivity in timing jitter.
5. M-ary modulation schemes have very good power efficiency. State whether True or False.
Explanation: M-ary modulation schemes have poor power efficiency, but they have a better bandwidth efficiency. An 8-PSK system requires a bandwidth that is 3 times smaller than a BPSK system, whereas its BER performance is very worse since signals are packed more closely in the signal constellation.
6. In M-ary PSK, the carrier ______ takes one of M possible values.
Explanation: In an M-ary PSK, the carrier phase takes one of the M possible values. The possible values of phase are θi=2(i-1)/M, where i=1,2,…….,M.
7. The constellation of M-ary PSK is _______ dimensional.
b) Does not exist
Explanation: The constellation of M-ary PSK is two dimensional. It is due to the presence of two basis signals. And the M-ary message points are equally spaced on a circle.
8. What is the radius of the circle in M-ary PSK on which message points are equaly spaced?
Explanation: The M-ary message points are equally spaced on a circle of radius √Es centred at the origin. Here Es is energy per symbol. Thus, MPSK is a constant envelope signal when no pulse shaping is used.
9. As the value of M _________ the bandwidth efficiency ________
a) Increases, same.
b) Increases, decreases
c) Increases, increases
d) Decreases, same
Explanation: The first null bandwidth of M-ary PSK signals decrease as M increases while Rb is held constant. Therefore, as the value of M increases, the bandwidth efficiency also increases.
10. The power efficiency of the M ary PSK decreases because of the _____
a) Freely packed constellation
b) Increment of bandwidth efficiency
c) Fixed null bandwidth
d) Densely packed constellation
Explanation: Bandwidth efficiency increases as the value of M increases. But at the same time, increasing M implies that the constellation is more densely packed. Hence the power efficiency or noise tolerance is decreased.
11. In QAM, the amplitude is _______ and phase is _______
a) Varied, constant
b) Varied, varied
c) Constant, varied
d) Constant, constant
Explanation: Quadrature amplitude modulation (QAM) is obtained by allowing the amplitude to also vary with the phase. Thus, the constellation consists of square lattice of signal points.
12. M-ary QAM signal have constant energy per symbol. State whether True or False.
Explanation: M-ary QAM does not have constant energy per symbol. It also does not have constant distance between possible symbol states. It reasons that particular values of M-ary QAM signal will be detected with higher probability than others.
13. In comparison to M-ary PSK, M-ary QAM bandwidth efficiency is _____ and power efficiency is ________
a) Identical, superior
b) Less, superior
c) Identical, identical
d) Superior, superior
Explanation: The power spectrum and bandwidth efficiency of QAM modulator is identical to M-ary PSK modulation. But, in terms of power efficiency QAM is superior to M-ary PSK.
14. The bandwidth efficiency of an M-ary FSK signal ________ with ________ in M.
a) Constant, increase
b) Increases, increase
c) Decreases, increase
d) Decreases, decrease
Explanation: The bandwidth efficiency of an M-ary FSK signal decreases with increase in M. Therefore, unlike M-PSK signals, M-FSK signals are bandwidth inefficient.
15. Power efficiency of M-ary FSK increases, since _________
a) Constellation is densely packed
b) M signals are non-orthogonal
c) Fixed null bandwidth
d) M-signals are orthogonal
Explanation: In M-ary FSK , all the M signals are orthogonal and there is no crowding in the signal space. Hence, power efficiency of M-ary FSK increases with M.
Sanfoundry Global Education & Learning Series – Wireless & Mobile Communications.
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